How to Fix a Green Pool in Spring (The SLAM Method Explained)
You pulled the cover off and found a swamp. A single bag of shock rarely fixes a truly green pool — the algae eats the chlorine faster than you add it. That's where SLAM comes in.
## What SLAM means
SLAM stands for **Shock, Level, And Maintain**. It was popularized by the pool owner community at Trouble Free Pool. Instead of one big dose, you raise free chlorine (FC) to a "shock level" and *hold it there* — topping up as often as needed — until the algae is completely gone.
The shock level depends on your cyanuric acid (CYA, stabilizer). A common guideline is FC at about **40% of your CYA**. So with CYA at 30 ppm you'd hold FC around 12 ppm; at 50 ppm, around 20 ppm. Liquid chlorine (bleach) is usually used because it doesn't add more CYA or calcium.
## SLAM step by step
1. **Test CYA and pH first.** Bring pH to roughly 7.2–7.6 so chlorine works well. 2. **Clean and circulate.** Skim debris, brush walls and floor, and run the pump 24/7. 3. **Raise FC to your shock level.** 4. **Test and top up** several times a day — morning, afternoon and evening if you can. Don't let FC drop below shock level. 5. **Brush daily and clean the filter** as pressure rises from dead algae. 6. **Stop when all three are true:** the water is clear, FC drops by 1 ppm or less overnight (tested at dusk and again at dawn), and combined chlorine is 0.5 ppm or less.
## SLAM vs. a normal shock
A normal shock is for routine maintenance — cloudy water or chloramine smell. SLAM is for visible algae (green, cloudy-green or black water), when you can't see the floor, or when your chlorine keeps disappearing overnight.
## How long it takes
Lightly green pools can clear in 2–4 days. A dark-green swamp can take a week or longer, especially if CYA is high.
## Common mistakes
- **Stopping too early.** Clear water doesn't mean the algae is gone — pass the overnight test first. - **Letting FC drop** below shock level between tests. - **Using stabilized products** like trichlor tablets or dichlor, which raise CYA and make SLAM slower. - **Not brushing**, or ignoring a clogged filter. - **Guessing doses** instead of testing.
Get your exact chlorine dose with the free [pool opening chemical calculator](/pool-opening-chemical-calculator), and see [how much chlorine it takes to fix a green pool](/how-much-chlorine-for-green-pool) for dosing by how green the water is.
Frequently asked questions
What does SLAM stand for?
SLAM stands for Shock, Level, And Maintain: raise free chlorine to a shock level based on your CYA and hold it there until the algae is completely gone.
What free chlorine level should I hold during SLAM?
A common guideline is about 40% of your cyanuric acid (CYA) level, for example around 12 ppm with CYA at 30 ppm. Test CYA first, since it sets the target.
How long does it take to SLAM a green pool?
Light green pools can clear in 2–4 days; very green or swampy pools can take a week or more, especially with high CYA.
How do I know when SLAM is finished?
When the water is clear, free chlorine drops by 1 ppm or less overnight, and combined chlorine is 0.5 ppm or less.
Can I SLAM with chlorine tablets or granular shock?
It's better to use liquid chlorine or bleach. Trichlor tablets and dichlor add stabilizer (CYA), which raises the chlorine level you need and slows the process.